Some twenty years ago Olivier Augé, one of the top speed French windsurfer, advised to sand horizontally near the head and turn vertically near the tip. This was supposed to optimize water flow and probably coped with the knowledge and technology of that time (remember the speed record was around 30 Knts). Frankly I wonder if sanding direction makes any sensible difference when the grit is fine enough. Select and Deboichet now sand their fins horizontally from head to tip, so the current (logical?) option seems to be parallel to the board.